This invention relates to control apparatus for a thyristor converter capable of changing by firing phase control the power to be supplied to a load which generates counter electromotive force, and particularly one which can compensate for the nonlinear relation between the set firing control angle of the thyristor converter and the load voltage (or load current) and hence deterioration of the response characteristic of the thyristor converter upon intermittent current flow.
As is well known, the thyristor converter is used to drive a load which generates a counter electro-motive force, such as a DC motor, AC motor etc.
When a motor is driven by the thyristor converter, the current flowing in the thyristor converter becomes continuous and intermittent depending on the condition of the load. When the current intermittently flows in the thyristor converter, the relation between the set firing control angle of thyristor converter and the load current (or load voltage) becomes nonlinear. This deteriorates the response characteristic of the control apparatus for the thyristor converter as is well known.
In order to compensate for the deterioration of the response characteristic of the control apparatus, the use of the following method was proposed. The difference between the firing control angles upon continuous and intermittent current flow where the average voltage of DC output of thyristor converter is the same, that is, a compensation control angle, is determined, and upon intermittent current flow the compensation control angle is added to the set firing control angle determined from the phase control signal, thereby producing the actual firing control angle. Thus, the nonlinear relation between the set firing control angle and load voltage (load current) can be compensated for, and hence the response characteristic of the control apparatus can be prevented from deterioration. This nonlinear relation compensating method is disclosed in Japanese Patent Laid-open Gazette No. 49979/1980 in which the applicant was Hitachi Ltd., and which was laid-opened on Apr. 11, 1980.
According to this method, the compensation control angle must be calculated in consideration of the main circuit constant which is determined mainly by the motor. The main circuit constants of motors manufactured even according to a specification have various values. If a set main circuit constant does not agree with the actual main circuit constant, the nonlinear characteristic compensation which will be made properly upon continuous current flow will be improperly made upon intermittent current flow. If, for example, a set main circuit constant is smaller than the actual one, continuous current will be misdetected as intermittent current, thus the compensation control angle being added to the set control angle results in overcompensation. On the contrary, if a set main circuit constant is larger than the actual one, intermittent current will be misdetected as continuous current. Also, in this case, even if intermittent current condition is properly detected to be intermittent, only a small angle is added for compensation, resulting in insufficient compensation.
Under excessive compensation, the thyristor transducer will produce a small DC output voltage in responsive to the phase control signal. The reduction of the DC output voltage results in small load current. Thus, the compensation control angle for compensation becomes large, or a positive feedback condition is brought about. In case of the insufficient compensation, the nonlinear characteristic is not compensated for even upon intermittent current flow and the compensation control angle is too small even in the region in which compensation is made.
Thus, in case of excessive compensation and insufficient compensation, optimum compensation for nonlinear characteristic cannot be made, and thus the response cannot be improved.